#include <fstream>

using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
int main()
{
    int n,m,lm=0,dm=0,dm1=0,lm1=0,li,di,smax=-1;
    in>>n>>m;
    for(int i=1;i<=m;i++)
    {
        in>>di>>li;
        if(lm==0)
            lm=li,dm=di;
        else if(lm1==0)
            lm1=li,dm1=di;

        else if(li-di>lm1-dm1)
            lm1=li,dm1=di;


        else if(li-di>lm-dm)
            lm=li,dm=di;


    }
    if(dm>dm1)
    out<<lm+lm1+dm-dm1;
    else out<<lm+lm1+dm1-dm;
    return 0;
}